CBS This Morning

Dozens of demonstrators gathered outside Dallas police headquarters Monday evening to demand justice for Botham Jean, the 26-year-old who was shot inside his own apartment last Thursday by off-duty police officer Amber Guyger. Guyger has been charged with manslaughter, but demonstrators say that's not enough. Her case now heads to a grand jury where her charges could be bumped up to murder, reports CBS News' Omar Villafranca.

According to an arrest warrant, Guyger told investigators that she went to what she thought was her third floor apartment. Instead, she went to Jean's fourth floor apartment directly above hers. Guyger says her door was "ajar" and saw a "large silhouette" inside. After giving "verbal commands that were ignored," she fired her handgun twice, striking Jean once in the torso. It wasn't until Guyger "turned on the ... lights" and "called 911" that she realized she was "at the wrong apartment."

"We will make certain that justice is done in this case," said Dallas County District Attorney Faith Johnson.

Attorneys for Jean's family say two witnesses told them details that contradict Guyger's account.

"They heard knocking down the hallway followed by a woman's voice that they believe to be officer Guyger saying, 'Let me in. Let me in,'" attorney Lee Merritt said.

The family's attorneys say one of the witnesses then heard gunshots followed by a man's voice.

"What we believe to be the last words of Botham Jean which was 'Oh my god, why did you do that?'" Merritt said.

Allison Jean wants to know what happened to her son. "I'm not satisfied that we have all the answers," Allison said.

"There is really no reason why a mother should have to wait until the conclusion of an investigation to know what happened to her child," said Daryl Washington, another one of the family's attorneys.

The Texas Rangers, who are leading an independent investigation, have been tightlipped about the case. Jean's family's attorneys say he was unarmed. They are expecting a large crowd for his funeral that's scheduled for Thursday at a church in Richardson, Texas.